Accurate detection of analytes in solutions, particularly biological fluids, is critical in several fields, including medical diagnostics, veterinary diagnostics, and food and drug safety. The innate turbidity of complex biological samples, such as blood, plasma, serum, urine, and bile, has made it difficult to develop reliable assays and devices for multiple analytes. Matrix-related interference as well as scattering of light by biocolloids hamper sensitive determinations of analytes where measurements are primarily restricted to turbidity.
Several existing assay methods for detection of analytes involve the use of antibody-antigen interactions (e.g. immunoassays). These assays usually involve tagging of an antibody with radioactive material (radioimmunoassays), conjugating the antibody to an enzyme (enzyme-linked immunosorbent assay or ELISA), or coloring the antibody using colorized latex or colored metallic nanoparticles. The detection of the antibody-antigen complex then occurs by determining the presence of the label (e.g. detecting radioactivity, measuring activity of the linked enzyme, or observing a color change). These assays have one or more of the following disadvantages: (1) entail time consuming multiple analysis steps, (2) require complicated, expensive machinery for readouts, (3) are limited to detection of single analytes, and (4) are limited to qualitative analyses. Thus, there is a need for additional methods to detect one or more analytes in a biological solution.
A colloidal gold test was used to study cerebrospinal pathology and liver dysfunction in 1912. Surface plasmon resonance (SPR) is a well known phenomenon occurring in metallic nanoparticle surfaces. The phenomenon describes a graded reduction in the intensity of the reflected light due to the molecular thickness of the metal surfaces when incident light strikes the surface at a certain angle.
Localized surface plasmon resonance (LSPR) is observed in mono-dispersed nanoparticles. The collective oscillations of their conduction electrons result in wavelength selective absorption and scattering of the incident radiation. The drawback of using LSPR for biological samples has always been interference due to non-specific reaction. Therefore, it would be desirable to develop a method for detecting analytes in biological solutions that utilizes the exquisite sensitivity of LSPR, but minimizes the interference from non-specific interactions.